Alloy-coated boiler part and method of welding self-fluxing alloy-coated boiler part

ABSTRACT

An alloy-coated boiler part is furnished with a melted coating of alloy material excellent in erosion/corrosion resistance when joined by welding, and free from thermal shock cracking. Super alloy coating ( 15 ) is applied over rapid temperature rise region width (C), where thermal shock cracking may occur at a welding operation, at end portions subjected to weld joint including the vicinity. Self-fluxing alloy coating ( 16 ) is applied on remaining regions other than the rapid temperature rise region width (C). While over a half proportion of each of the alloy coatings ( 15, 16 ) occupied by an Ni-enriched Ni—Cr component, the super alloy coating ( 15 ) has the contents of B and Si suppressed to equal or less than 0.1% and equal or less than  0.5 %, respectively, and in the self-fluxing alloy coating ( 16 ), the content of each of B and Si is in the range of 1 to 5%.

TECHNICAL FIELD

The present invention relates to a boiler part such as a tube part(hereinafter, referred to as “boiler tube”) constituting a heat transfertube in various types of boilers, or a plate material-tube materialcomposite panel (hereinafter, referred to as “boiler furnace panel”)constituting a furnace housing with a cooling-water passage, and morespecifically to a boiler part with alloy coating for improvingdurability and suitable to the welding, and a method of welding analloy-coated boiler part.

BACKGROUND ART

Firstly, taking an example of boiler tube, in the past days when theoperation temperature of the boiler was lower than that in these days,and erosion/corrosion environment inside the furnace was not so severe,the steel tube for boiler (low alloy steel tube) was commonly usedwithout coating taking into consideration of high temperature usabilityand mechanical characteristics. Although stainless tube and titaniumtube were also used for the application of requiring corrosionresistance, such uses were not common because of the high costs.

In recent years, boilers recovering and utilizing refuse incinerationheat have increased, problem of erosion (wear) is caused by combustionash dust. In order to solve this problem, boilers with a specificationapplying thermal spray coating of self-fluxing alloy (first alloymaterial) with high erosion resistance are starting to be commonly used.

However, the above described self-fluxing alloy coating is the coatingwhich is left as thermal-sprayed by, for example, HVOF (High VelocityOxygen Fuel) thermal spraying apparatus (that is, the coating is poroushaving pinholes reaching base metal herein after referred to as“unmelted coating”); a self-fluxing alloy coating subjected to themelting process after thermal spraying is rarely used although meltedcoating is commonly used in other application (for example, rollers formetal sheet processing line) (which is modified from the porous coatinginto the dense coating, and provided with sufficient environmentblocking function without pinholes, and is the example of the “meltedcoating”).

The reason why the application of the melted coating of the self-fluxingalloy for the boiler tube is uncommon is that an extraordinary difficultwork is required because the thermal shock cracking easily occurs to theboiler tube on its melted coating under rapid local temperature rise atthe time of the welding operation when the boiler tube is welded toconnect, the whole tubes have to be preheated inside the furnace, thenconnected by welding at high temperature.

However, as for the boilers in recent years, the problems concerning notonly erosion but also corrosion are becoming important together with thedemand for high-temperature burning to make the exhaust air harmless,thus melted coating application to the boiler is increasingly desired inthe form of provision of prefabricated coating portion.

As one example thereof, a constitution that adopted the melted coatingof the self-fluxing alloy to the boiler tube is disclosed in the patentdocument 1 (Japanese Patent Application Laid-Open (JP-A) No.H10-170194). There the constitution provides a non-coated portion ofabout 50 mm at an end portion of the boiler tube, excluded from thethermally sprayed coating where the non-coated portion is used as a partfor the joint (the patent document 1, page 3, the fourth column, lines15 to 16). Further, a process of fitting a protector member on the abovenon-coated portion is added instead of the coating, after the welding(Id. lines 24 to 26). The above process requires a special order of theprotector member (for example, the protector member made of alumina)having high erosion resistance, or fitting operation inside narrowboiler. Therefore, the process results in higher cost in material andwork, and also requires front loaded procurement of the materials.

As for another approach, one may imagine a method that adopts welding toconnect prefabricated boiler tubes having unmelted coating of aself-fluxing alloy, at the construction site, and then apply the meltingprocess by an induction heating or the like at that location. However,it is practically impossible due to the narrow space or difficulty inheating of the interfaced portion with the other members.

By the way, since, reheating-crack occurs unless the entire simultaneousmelting or one direction melting is performed in the melting process ofthe self-fluxing alloy, the induction heating is indispensable for suchan execution at the construction site.

Next, in the case of the boiler furnace panel, since it is compositeconstitution in which tube materials and plate materials are arrangedalternately as described above, or since it has large dimension (forexample, 0.5 m×6 m), use of a practical supplemental membercorresponding to the above protector is more difficult. Further there isthe problem of the complicated shape distortion associated with themelting process after the thermal spraying (see the patent document 2 orthe patent document 3). Thus utilization of the prefabricatedmelt-coating product itself has been difficult to consider under suchcircumstances.

Patent Document 1: JP-A No. 10-170194

Patent Document 2: JP-A No. 2001-4101

Patent Document 3: JP-A No. 2000-329304

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The first problem is to provide an alloy-coated boiler part that iscoated with melted coating of alloy material with high erosion andcorrosion resistance at all regions to be protected and that is freefrom thermal shock cracking even when joined by the welding.

Further, the second problem is to provide a method of weldingself-fluxing alloy coated boiler parts to which the melted coating ofthe self-fluxing alloy has been applied at all regions, where the methodenables the welding for joining the boiler part and so on free fromthermal shock cracking.

Means for Solving the Problems

The alloy coated boiler part according to the present invention has beendevised for solving the above mentioned first problem and ischaracterized in that

an alloy coated boiler part used by being welded to be joined, isconstituting of a base metal and an alloy coating applied thereon, wherethe alloy material is comprising of an Ni (Nickel)-enriched Ni—Cr(chromium) component with over a half proportion of the alloy material,

a melted coating of said alloy material further comprising of reducedcontents of B (Boron) and Si (Silicon) as melting point loweringelements with 0.1 weight % or less B and 0.5 weight % or less Si (thesecond alloy material), is applied over end portions subjected to weldjoint including the vicinity thereof where thermal shock cracking mayoccur due to a rapid temperature rise at the welding operation, and

a melted coating of said alloy material further comprising of B and Siwith each content in the range of 1 to 5 weight % (any of 1% or more and5% or less) (the first alloy material), is applied (desirably byafter-thermally sprayed melt-coating) on any remaining regions otherthan areas subject to the rapid temperature rise.

By the way, as for a forming means of the melted coating composed of theabove second alloy material, there can be exemplified the weldbuilding-up (one layer building-up if importance is given to cost, ortwo layers building-up if importance is given to component purity),although not limited to such method.

Further, a method of welding a self-fluxing alloy coated boiler partaccording to the present invention is devised for solving the abovementioned second problem, being a method of welding self-fluxing alloycoated boiler part where a melted coating (desirably after-thermallysprayed melt-coating) composed of a self-fluxing alloy material occupiedby an Ni-enriched Ni—Cr component over a half proportion of the alloymaterial and further occupied by B and Si in the range of 1 to 5 weight% respectively (the first alloy material) is applied to a base metal,the method is characterized by comprising the steps of:

forming a gradation preheated region, around the end portion subjectedto the welding, upon applying preheating process having a heatingpattern where an amount of temperature raising gradually reduces inwardfrom the end portion by using slow heating condition with a speed oftemperature raising at said end portions of 2 to 10° C./sec; and

performing a welding operation of said end portions continuously(desirably, with the second alloy material occupied by an Ni-enrichedNi—Cr component over a half proportion of the alloy material and havingthe reduced contents of B and Si with 0.1 weight % or less for B and 0.5weight % or less for Si.)

By the way, in the above welding operation, also a weld building-upoperation for the end portion of single part and weld joint of the endportions of neighboring parts are included.

EFFECT OF THE INVENTION

The alloy coated boiler part of the present invention is constitutedsuch that the melted coating composed of the alloy material havingfusibility with melting point lowering elements sufficiently mixed (thefirst alloy material) has been applied at the most area except for theend portions. Thus, manufacture in a factory with high productivity andlow cost same as those of ordinary thermal spraying-melting process canbe performed. On the other hand, the end portion region has no choicebut to depend on high cost implementation other than the thermalspraying such as the weld building-up or the like with low productivity.However, it is possible to manufacture with a low cost as a whole, sincethe region area is small.

The melt coating of the end portion region keeps the mixing amount of B,Si to a minimum, that is, the thermal coating is composed of the secondalloy materials. Therefore, there is no fusibility, thus highproductivity cannot be desired. However, instead of this, the endportion region is free from the thermal shock cracking sensitivity whichthe alloy material with sufficiently high concentration of B and Siexhibits.

With respect to durability, since the both coatings are the meltcoatings, the environment blocking property is sufficient, further, alsothe corrosion resistance of the coating itself is significantlyexcellent due to high Ni—Cr composition. With respect to erosionresistance, the coating for the most region being composed of largeamount of B, Si excels. However, since the difference is not serious, atreatment of adding difference to an initial coating thickness iscapable of approximately equalizing the erosion resistance.

As described above, the first problem is solved.

According to the method of welding the self-fluxing alloy coated boilerpart, the thermal shock cracking of the self-fluxing alloy meltedcoating caused by rapid temperature rise at the time of the welding, isnot generated by providing “gradation preheated region” in such a way asto involve filler metal applied region. Because, it is possible tominimize the amount of rapid change of temperature over the entireregion accompanied with the rapid temperature rise.

The above described welding method is useful when a rapid preparation ofthe alloy coated boiler part is required due to an unexpected conditionsuch as dimension adjustment in the boiler construction site. Forexample, it is possible to prepare the alloy coated boiler partequivalent to the product, having been adjusted at the constructionsite, and free from the thermal shock cracking even when supplied forwelding for joining, in following procedures; the self-fluxing alloycoated boiler part with melted coating manufactured in the factory is atfirst adjusted in dimension; then the coating of the self-fluxing alloy(the first alloy material) of the end portion is removed; and further,the weld building-up is performed to the end portion with the alloymaterial of the composition for the end portion (desirably, the secondalloy material). Or it is possible to perform repair of the self-fluxingalloy coating by applying the above described pre heating-weldbuilding-up to the coating defect portion (exfoliation portion or thelike) of the member on which the self-fluxing alloy coating (the thermalsprayed coating or un-melted coating) has been applied.

Here, the gradation preheated region prepared before the building-upimplementation is capable of being formed as gradual temperaturedistribution by performing induction heating using the solenoid coil inwhich winding density is gradually changed along the axis line directionso as to generate a gradient of heat input density along the axis linedirection and by incorporating procedure of swinging a solenoid coilappropriately in the axis line direction.

The above preheating process is somewhat troublesome work. However, itis possible to implement reasonably in a narrow space inside the boiler,since the object region is an only narrow range, and further, a minimumscale of a high frequency power supply is required. By the way, thepreheating in this welding method may be performed efficiently by theinduction heating as described above. However, being not limited tothis, it may be substituted by other heating method, such as, forexample, gas heating.

Above described procedure is significantly effective in rapidly formingand saving beforehand at the boiler construction site the alloy coatedboiler part that are easily jointed by welding.

The above described welding method, further, is also useful whenintended to perform welding joint of the self-fluxing alloy coatedboiler part having melted coating and cut in actual dimensions withoutgenerating thermal shock cracking at all events at that constructionsite in stead of preparing of the alloy coated boiler part; thus itbecomes a temporary measure of corresponding to the boiler parts with auniform alloy coating instead of provision of the alloy coated boilerpart.

In the method of welding the self-fluxing alloy coated boiler part, itis desirable that the alloy material of the composition for the endportion (the second alloy material) is set to a filler metal whileconsidering quality requirement (erosion/corrosion resistance) after thewelding.

As described above, the second problem is solved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is the entire plan view showing structure of an alloy coatingboiler part according to an embodiment of the present invention;

FIG. 1( b) is the entire front view/end elevational view showingstructure of an alloy coated boiler part according to an embodiment ofthe present invention;

FIG. 1( c) is an enlarged plan view of the major portion showingstructure of an alloy coated boiler part according to an embodiment ofthe present invention;

FIG. 1( d) is a principal part enlarged front view/end elevational viewshowing structure of an alloy coated boiler part according to anembodiment of the present invention;

FIG. 1( e) is A-cross section enlarged view showing structure of analloy coated boiler part according to an embodiment of the presentinvention;

FIG. 1( f) is a plurality of joined state plan view showing structure ofan alloy coated boiler part according to an embodiment of the presentinvention;

FIG. 2( a) is the entire plan view showing a manufacturing process ofthe alloy coated boiler part;

FIG. 2( b) is the entire front view/end elevational view showing amanufacturing process of the alloy coated boiler part;

FIG. 3( a) is an enlarged plan view of the major portion showing amanufacturing process of the alloy coated boiler part;

FIG. 3( b) is an enlarged front view/end elevational view of the majorportion showing a manufacturing process of the alloy coated boiler part;

FIG. 4( a) is an enlarged plan view of the major portion showing amanufacturing process of the alloy coated boiler part;

FIG. 4( b) is a major portion enlarged front view/end elevational viewshowing a manufacturing process of the alloy coated boiler part;

FIG. 4( c) is the entire plan view showing a manufacturing process ofthe alloy coated boiler part;

FIG. 4( d) is the entire front view/end elevational view showing amanufacturing process of the alloy coated boiler part;

FIG. 4( e) is the entire plan view showing a manufacturing process ofthe alloy coated boiler part;

FIG. 4( f) is the entire front view/end elevational view showing amanufacturing process of the alloy coated boiler part;

FIG. 5( a) is an enlarged plan view of the major portion showing amanufacturing process of the alloy coated boiler part;

FIG. 5( b) is a major portion enlarged front view/end elevational viewshowing a manufacturing process of the alloy coated boiler part;

FIG. 6( a) is a cross sectional view showing the weld joint portionbefore the welding of welding process of the alloy coated boiler part;

FIG. 6( b) is an enlarged cross sectional view showing the weld jointportion after welding of welding process of the alloy coated boilerpart;

FIG. 7( a) is a plan view of a boiler tube showing a manufacturingprocess of the alloy coated boiler part according to the otherembodiment of the present invention;

FIG. 7( b) is a plan view/end elevational view of a boiler tube showinga manufacturing process of the alloy coated boiler part with respect tothe other embodiment of the present invention;

FIGS. 7( c) to 7(h) are plan views of a boiler tube showing amanufacturing process of the alloy coated boiler part according to theother embodiment of the present invention;

FIGS. 8( a) and 8(b) are plan views of a boiler tube showing amanufacturing process of the alloy coated boiler part according to theother embodiment of the present invention; and

FIGS. 9( a) and 9(b) are plan views of major portions of a boiler tubeshowing a manufacturing process of the alloy coated boiler partaccording to the other embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Constitution of one embodiment of the alloy coated boiler part of thepresent invention will be described while referring to the drawings.FIG. 1( a) to FIG. 1( e) show structure of a boiler furnace panel 10 asthe specific example of the alloy coated boiler part, in which FIG. 1(a) is the entire plan view; FIG. 1( b) is the entire front view/endelevational views; FIG. 1( c) is an enlarged plan view of the majorportion; FIG. 1( d) is a principal part enlarged front view/endelevational view; FIG. 1( e) is cross section enlarged view; and FIG. 1(f) is a plurality of joined state plan view.

The boiler furnace panel 10 is one in which a super alloy coating 15(the second alloy material coating) and a self-fluxing alloy coating 16(the first alloy material coating) are thermally sprayed to a steelpanel 11 (a base metal) at a factory beforehand, on the occasion ofassembling of the furnace housing, neighboring end portions each otherare weld joined while overlapping a plurality of panels at theconstruction site. That is, a steel panel 11 (plate material-tubematerial composite panel) is constituted such that, in order to form abase unit of furnace housing with a cooling water passage, a tubeportion 12 (tube material) forming a cooling water passage and a plateportion 13 (plate material) forming a joint portion are arrangedalternately and these are joined tightly with weld joint or the like,further, for erosion/corrosion resistance, at the one side (portion tobe protected) of the housing, being furnace inner wall, except for endportions for weld joint, weld sprayed coating composed of the alloymaterial is formed on the entire area.

In the thermal spraying, the super alloy coating 15 and the self-fluxingalloy coating 16 are used separately for reduction of the cost ofmaterial and the cost of construction, in which the super alloy coating15 is carried out at belt shaped region (rapid temperature rise region)in the end portions of a tube end side provided to weld joint portion 20among panel end portions, while the self-fluxing alloy coating 16 iscarried out at remaining region among region to be protected. The rapidtemperature rise region is a region in which the thermal shock crackingmay occur in self-fluxing alloy coating at the time of the weldoperation, since the rapid temperature rise region width C differsdepending on a steel material shape or coating thickness or the like, itcannot be described sweepingly, but, in the boiler furnace panel, it maybe about 15 to 50 mm (any of 15 mm or more and 50 mm or less).

Of course, with respect to the end portion of the plate portion 13, thesuper alloy coating 15 is formed up to inner portion exceeding it. Thatis, the super alloy coating 15 enters into not only the rapidtemperature rise region but also the remaining region which is a regionexceeding it while extending up to about two times of the rapidtemperature rise region width C. Further, at that position, a thinnotching 14 with a width of about 0.5 to 2 mm is formed. This secureswarping margin for weld aligning of the tube portion 12, and may extendto a degree of several times of the rapid temperature rise region widthC together with the super alloy coating 15 of the end portions of theplate portion 13 depending on thickness of the tube portion 12. At endportion surface of the tube portion 12, large chamfer/taper to become awelding groove is provided.

The material of the super alloy coating 15 is occupied by an Ni-enrichedNi—Cr component over a half proportion of the alloy material, however,contents of B and Si being melting point lowering elements aresuppressed such that B is 0.1% or less and Si is 0.5% or less foravoiding a thermal shock cracking at the time of the welding operation.As standards for stipulating such alloy material, in Japan, there arelisted JIS G 4901 for bar material, or JIS G 4902 for plate material, asInternational standards, there are listed ISO 4955, or ISO 9723.Thickness of the super alloy coating 15 is about 1.2 to 3.0 mm.

The material of the self-fluxing alloy coating 16 is occupied by anNi-enriched Ni—Cr component over a half proportion, and contents of B,Si are set to 1 to 5% (weight ratio) respectively in order that steeprise of material cost is made to suppress and efficient implementationis brought by the thermal spraying process and the melting process. Asthe alloy material, in Japan, there is listed Nickel self-fluxing alloymaterial of composition stipulated in a JIS H 8303. In other countriesor areas, there is listed Nickel self-fluxing alloy material ofcomposition stipulated in International standards ISO 14920. By the way,as the self-fluxing alloy material, though high price, Co (cobalt) basedself-fluxing alloy material or WC (tungsten carbide) mixed self-fluxingalloy material may be used as the need arises. Thickness of theself-fluxing alloy coating 16 is about 1.0 to 2.0 mm.

A manufacturing process concerning the boiler furnace panel 10 (alloycoated boiler part) of the embodiment will be described referring to thedrawings. FIG. 2( a) is the entire plan view; FIG. 2( b) is the entirefront view/end elevational view; FIG. 3( a) is an enlarged plan view ofthe major portion; FIG. 3( b) is an enlarged plan view of the majorportion; FIG. 4( a) is an enlarged plan view of the major portion; FIG.4( b) is a major portion enlarged front view/end elevational view; FIG.4( c) is the entire plan view; FIG. 4( d) is the entire front view/endelevational view; FIG. 4( f) is the entire front view/end elevationalview; FIG. 5( a) is an enlarged plan view of the major portion; and FIG.5( b) is a major portion enlarged front view/end elevational view.

The manufacture of the boiler furnace panel 10, in short, is one inwhich a steel panel 11 is taken to as a base metal, a super alloycoating 15 is formed on an one side of the steel panel 11, after that,also a self-fluxing alloy coating 16 is formed thereon, and further, endportion shape is finished, being performed in a factory.

The steel panel 11 (see FIG. 2) is preferable in the same asconventional one in which the tube portion 12 and the plate portion 13composed of steel materials are joined alternately by the weld joint orthe like. In the case of ordinary boiler furnace panel, size of thesteel panel 11 is in the dimension that length is about 4000 to 6000 mmand width is about 400 to 500 mm, diameter of the tube portion 12 isabout 60 to 75 mm, thickness of the tube portion 12 is about 5.0 to 7.0mm, and thickness of the plate portion 13 is about 5 to 7 mm.

Formation of the supper alloy coating 15 concerning the end portion ofthe steel panel 11 (see FIG. 3), in the tube portion 12, is performed tosomewhat wider range than the rapid temperature rise region width C,while, in the plate portion 13, being performed to a portion comefurther inwardly. By the way, in some cases, the tip of about 50 to 150mm remains without performing coating process for fixing and keeping ofthe work during operation, however, at that case, it is cut off at thetime of end portion shape finishing. Implementation work of the superalloy coating 15 is performed with the weld building-up, the secondalloy material being made into a wire material is suited as a fillermetal, above all, in Japan, the super alloy materials stipulated in JISG 4901-NCF 625, and JIS G 4902-NCF 625 are suitable, while in othercountries or areas, it is possible to select corresponding article fromthe super alloy materials stipulated in international standards of ISO4955 or ISO 9723 or the like.

Formation of the self-fluxing alloy coating 16 to the remaining regionof the rapid temperature rise region (see FIG. 4) is performed in theorder of masking of the super alloy coating 15, thermal spraying of theself-fluxing alloy, and melting process of the self-fluxing alloy. Bythe way, although detailed description is omitted, surface cleaningprocess such as shot blast and the like is performed appropriately.Masking of the super alloy coating 15 is performed using, for example, ablocking plate such as metal thin plate or the like, or heat-resistantmasking tape. Further, the masking is performed such that the superalloy coating 15 is partially overlapped on the self-fluxing alloycoating 16 in order that a base metal is not exposed from a gap formedbetween the super alloy coating 15 and the self-fluxing alloy coating16. In such overlapped portion, it is preferable to add taper inthickness of the both coating so that a sudden difference in level doesnot emerge.

Since the thermal spraying and the after melting process for theformation of the self-fluxing alloy coating 16 is preferably performedin such a way as coating formation process to a conventional articlewith no super alloy coating 15 (for example, see the patent document 2),although detailed description is omitted, it is performed with ordinaryprocedure by using known apparatus. That is, thermal spraying of theself-fluxing alloy is performed efficiently with ordinary thermalspraying method by using known thermal spraying apparatus. As for thethermal spraying material of the self-fluxing alloy coating 16, thefirst alloy material powdered is suited, above all, in Japan, nickelself-fluxing alloy material corresponding to JIS H 8303-SFNi 4 ispreferable, in other countries or areas, it is possible to selectcorresponding article from nickel self-fluxing alloy materialsstipulated in the international standards of ISO 14920.

Further, basically, the melting process of the self-fluxing alloy isperformed efficiently in one direction with ordinary movement heatingmethod by using known high frequency induction heating apparatus.

However, different from the conventional method, sufficient thermalspraying process is made to bring under the condition of one directionalmovement to the self-fluxing alloy coating 16 thermally sprayed to theplate portion 13 (see patent documents 2, 3).

In the case of coating with the above alloy materials, it is preferableto set thickness ratio between the super alloy coating 15 and theself-fluxing alloy coating 16 into 1.2 to 2.0:1. This is becausewear-resistance of the self-fluxing alloy coating is superior to thesuper alloy coating, thus, by presetting the coating thickness ratio ofthe above range depending on conditions, wear-resistant service life ofthe both coatings are equalized. That is, in less than 1.2:1, wear ofthe super alloy coating 15 precedes, there is a fear that remainingself-fluxing alloy coating 16 becomes useless, on the other hand,exceeding 2.0:1, inversely wear of the self-fluxing alloy coating 16precedes, there is a fear that remaining super alloy coating 15 becomesuseless.

After formation of the alloy coatings 15, 16 on one side of surface,un-coated portion of the tip of the steel panel 11 is cut off (see FIG.5). This is done by a plasma cutting or the like, simultaneously oranother time, a notch 14 is formed. Further, chamfering process iscarried out at the tip of the tube portion 12 while preparing for weldjoint for another boiler furnace panel 10 (see FIGS. 1( c), 1(e)).

Thus, when terminating the finishing of the end portion, one of theboiler furnace panel 10 is completed. Further, similarly, when theboiler furnace panel 10 is efficiently manufactured one after another,these are accumulated to be kept in a factory or warehouse or the like.

Use state concerning the boiler furnace panel 10 (alloy coated boilerpart) of the embodiment will be described referring to the drawings.Since a plurality of boiler furnace panels 10 are assembled into aboiler furnace with the plurality of panels with weld-joining, here,particularly there will be described weld process of the boiler furnacepanels 10 each other. FIG. 6( a) is a cross sectional view showing theweld joint portion before the implementation of welding process of thealloy coated boiler part; and FIG. 6( b) is an enlarged cross sectionalview showing the weld joint portion after welding of welding process ofthe alloy coated boiler part. Further, FIG. 1( f) is a plan view showinga joined state of a plurality of the boiler furnace panels 10.

Weld joint process comprises positioning (aligning) process, tube endportion weld joint process and plate portion weld joint process, underthat order, treatment of each process is applied to weld joint portion20 of a pair of weld object boiler furnace panels 10. In some cases,this is partially performed in assembling factory, however, this isperformed finally in a boiler construction site.

Firstly, in the positioning process (see FIG. 6( a)), both boilerfurnace panels 10 are fixed in the state that the tip end portions oftube part 12 to become the weld joint portion 20 are caused to face.Then, if there is position deviation in each tube end portions facedstate, and the position deviation is slight one which is generated byformation of the alloy coatings 15, 16, position adjustment of the facedtube end portions is performed by knocking a small wend portion into thenotch 14 and the like.

Next, in the tube end portion weld joint process (see FIG. 6( b)),annular welding is performed from a tube inner surface side to a tubeouter surface side for several times for no generation of voids, and forno overheating. In the shown example, since being separated in fivelayers, the welding starts from the annular welding of a super alloywelding layer 21 facing on hollowness of the tube, followed by theannular welding of a super alloy welding layer 22 buried with the tubethickness, further, three columns of super alloy welding layers 23, 24and 25 exposed on the tube outer periphery are carried out sequentiallyevery one round. As for the filler metal used in the tube end portionwelding process, the second alloy material as being the super alloycoating 15 is desirable in the point of balance of corrosion resistance,and wear resistance to the self-fluxing coating. The super alloy coating15 carried out using the second alloy material has no thermal shockcracking sensitivity, therefore, the welding is performed easily andaccurately without injuring the coating. Further, as for theself-fluxing alloy coating 16 ahead of it, though having the thermalshock cracking sensitivity, since the super alloy coating 15 covers therapid temperature rise region width C, also, it is not necessary forconcerning a generation of cracking.

Finally, in the plate portion weld joint process, although illustrationis omitted, an application plate with a size for covering at least bothnotches 14 is welded while extending over the plate portion 13 of theboth boiler furnace panels 10. The welding of the application plate isperformed from the other side (that is, un-formed surface of the superalloy coating 15, furnace outer wall surface or unprotected surface).The notch 14 is thin and further change of the application plate iseasy, therefore, in some cases, process is terminated by leaving innerside of the notch 14 in the state of the base metal as it is, that is,the inner side of the notch 14 is left such that the steel panel 11 isexposed, however, it may be preferable to fill up in the notch 14 withthe welding while using the second alloy material as being the superalloy coating 15 as the filler metal. Further, as for the plate portion13, different from the tube portion 12, since the repair after theboiler construction is easy to perform (even the repair being performedfrom outside the boiler, rough object can be achieved), it may beadopted the procedure that, after realizing long service life whilethickening a panel end portion plate or plate thickness of theapplication plate, the alloy coating is omitted.

Thus, the boiler furnace panel 10 is joined to one after another (seeFIG. 1( f)), the furnace housing with cooling water passage iscompleted.

In such a furnace housing, the entire region concerning the entiresurface of inner wall, or portion to be protected of the inner wallsurfaces, are covered with close alloy coating, thereby,erosion/corrosion resistance is improved significantly.

Next, processes concerning one embodiment of the method of welding theself-fluxing alloy coated boiler part of the present invention will bedescribed referring to the drawings. FIG. 7( a) is a plan view of aboiler tube; FIG. 7( b) is a plan view/end elevational view of a boilertube; FIGS. 7( c) to 7(h) are plan views of a boiler tube; and FIGS. 8(a) and 8(b) are plan views of a boiler tube.

Here, there will be described supplying method of the boiler tube at theconstruction site which the boiler tube is not prepared beforehand.Provided that the steel boiler tube 70 subjected to the self-fluxingalloy coating on outer circumferential surface being a portion to beprotected (see FIGS. 7( a), 7(b)) can be obtained rapidly, however, itslength does not agree with the assembling portion (see FIG. 7( c)).Further, the self-fluxing alloy coating is alloy material occupied byNi-enriched Ni—Cr component over a half proportion of the alloymaterial, and in which B and Si are mixed in the range of 1 to 5%respectively, that is, the self-fluxing alloy coating is the first alloymaterial (above described material of the self-fluxing alloy coating 16)having the thermal shock cracking sensitivity, when it is welded in thestate that self-fluxing alloy coating 16 is left as it is, the weldingbecomes not desired state.

Accordingly, an adaptation portion with required length is made toremain by cutting off a surplus portion 71 from the boiler tube 70 (seeFIG. 7( d)), for after weld joint (see FIG. 8( b)), firstly, performedis the welding of the weld building-up at both end portions of theadaptation portion 72 being the adoption portion (see FIGS. 7( e) to7(h), and FIG. 8( a)). The process order of the welding is described indetail, firstly, the self-fluxing alloy coating is removed from the bothend portions 73, 74 within the adaptation portion 72 (see FIG. 7( e)).The base metal exposed width of the end portions 73, 74 are the same asthe above described rapid temperature rise region width C, though cannotbe described indiscriminately, being about 15 to 50 mm.

After that, an induction coil 75 configured such that the coil 75 iswound in a solenoid shape and its winding pitch increases monotonously(see FIG. 7( f)) is made to connect to the high frequency inductionheating apparatus; the end portion 73 is inserted to be fitted withfreedom into the coil 75 (see FIG. 7( g)), to perform preheating processdue to induction heating. At this time, an induction coil 75 which has alength of 2 to 3 times of base metal exposed width in the end portion 73is used, where rough portion of the winding pitch is set to anintermediate portion side of the adaptation portion 72 while denseportion of the winding pitch is set to the end portion 73 side betweenboth end portions of the induction coil 75. Further, energizingcondition of the induction coil 75 set on the high frequency inductionheating apparatus is made a gradual speed heating condition that atemperature raising speed in the end portion 73 becomes 2 to 10° C.

Then, the adaptation portion 72 is preheated slowly with heating patternwhere the amount of temperature rise gradually decreases inwardly fromthe end portion 73. Subsequently, if the temperature of the maximumtemperature portion becomes 450 to 600° C., the heating ends. By thismeans, “gradation preheated region” with no sudden change in a respectof space in axial direction and in respect of elapse of time is formed,therefore, consecutively, that is, swiftly before being cooled, thesuper alloy coating 76 is formed at the end portion 73 of the adaptationportion 72 (see FIG. 7( h)). The formation is performed such that, withthe weld building-up, no base metal exposed surface is made to leave atthe position between the self-fluxing alloy coating and the super alloycoating 76.

Further, in the weld building-up, the second alloy material (abovedescribed the material of the super alloy coating 15) with no thermalshock cracking sensitivity is used as the filler metal. That is, thealloy material occupied by an Ni-enriched Ni—Cr component over a halfproportion of the alloy material and contents of B and Si being meltingpoint lowering elements are suppressed such that B is 0.1% or less andSi is 0.5% or less is taken to as the filler metal, and the welding isperformed.

In such a welding, previous formation is the self-fluxing alloy coating,and after formation is the super alloy coating, however, the welding iscarried out within the “gradation preheating region”, therefore, thereis no chance of cracking the self-fluxing alloy coating.

Thus, the adaptation portion 72 of the boiler tube 70 is constitutedsuch that, as for the end portion 73 being an object region of the laterweld joint, the super alloy coating by the second alloy material isformed extending over the rapid temperature rise region, and, at theinner remaining region from the super alloy coated region, theself-fluxing alloy coating is left. Then, if the same welding isperformed with respect to the remaining end portion 74, thecorresponding boiler tube 80 to the boiler furnace panel 10 describedabove is completed (see FIG. 8( a)). Also the other tubes 81, 82 joinedto the boiler tube 80 are prepared similarly at the construction site ifnecessary, and then being kept temporarily.

Then, at the time of the weld joint, the boiler tubes 80, 81 and 82 asbeing joining object are arranged at the construction site, afterperforming the required pipe end portion treatment such as formation ofthe welding grooves, the boiler tubes 80, 81 and 82 are made to fix withthe condition that the tips of the boiler tubes 80, 81 and 82 areoppositely faced to each other at the weld joint portions 83, 84 (seeFIG. 8( b)). Then, the weld joint of the pipe end portions is performedto the respective weld joint portions 83, 84 while using the same fillermetal used for the weld building-up of the super alloy coating 76. Thetube end portion weld joint of the boiler tube 80, like the tube endportion weld joint of the tube portion 12 described above, can beperformed easily without a concern of the thermal shock cracking of theself-fluxing alloy coating, and the preheating is not necessary.

Processes concerning the other embodiment of the method for welding theself-fluxing alloy coated boiler part of the present invention will bedescribed referring to the drawings. FIGS. 9( a) and 9(b) are plan viewsof major portions of a boiler tube.

Here, there will be described the weld joint method when it becomesnecessary to perform the weld joint of the steel boiler tubes 90subjected to the self-fluxing alloy coating on the outer circumferentialsurface at the construction site in a hurry. Here, also, since theself-fluxing alloy coating is alloy material occupied by Ni-enrichedNi—Cr component over a half proportion of the alloy material, and inwhich B and Si are mixed in the range of 1 to 5% respectively, that is,the self-fluxing alloy coating is the first alloy material (abovedescribed material of the self-fluxing alloy coating 16) having thethermal shock cracking sensitivity, when it is welded in the state thatself-fluxing alloy coating 16 is left as it is, the welding becomes notdesired state, there is performed formation of the “gradation preheatedregion” by using the high frequency induction heating apparatus.

Reasonably, the preheating of this case, different from the abovedescribed respective embodiments, is performed immediately before theweld joint. Further, in many cases, the both boiler tubes 90 of thejoining object are already fixed at the construction site, and it isdifficult to separate the both end portions 91. For that reason, it ispreferable that the end portion 91 is subjected to pre-processing offorming the weld grooves, while, for the induction coil 92, one-turnarticle capable of acting in deployment, and put on and taken off isadopted (see FIG. 9( a)). Then, when performing the heating forformation of the “gradation preheated region” while high frequencyenergizing the induction coil 92, the induction coil 92 is caused toreciprocate.

The reciprocating movement is performed many times until the temperatureof the maximum temperature portion arrives at 450 to 600° C. Further,the heating is performed at the region of the end portion 91 in gradualspeed, to the contrary, with being separated from the end portion 91 inrapid speed, in order that the heating pattern becomes a pattern wherethe amount of temperature rise gradually decreases with heading towardthe both directions, with the end portion 91 of the weld joint object asthe center, that is, the heating pattern becomes a pattern where theamount of temperature rise gradually decreases with heading inwardly inlongitudinal direction of the boiler tube 90 from the end portion 91.The reciprocating distance, at one side, that is, with respect to oneboiler tube 90, of about 15 to 50 mm is secured inwardly from the endportion 91. Further, in this case also, energizing condition of theinduction coil 92 set to the high frequency induction heating apparatusis set to gradual speed heating condition such that the temperatureraising speed in the end portion 91 becomes 2 to 10° C./sec.

Then, also in this case, “gradation preheated region” with no suddenchange in respect of space in axial direction and in respect of elapseof time is formed, therefore, consecutively, faced end portions 91 ofthe boiler tube 90 are made to perform weld joint swiftly (see FIG. 9(b)). The weld joint of the weld joint portion 93, like the tube endportion weld joint of the tube portion 12 described above, is performedin which the second alloy material (above described material of thesuper alloy coating 15) with no thermal shock cracking sensitivity istaken to as the filler metal. That is, in this case also, the fillermetal is the alloy material occupied by an Ni-enriched Ni—Cr componentover a half proportion thereof and contents of B and Si are suppressedsuch that B is 0.1% or less and Si is 0.5% or less, however, since thegradation preheating precedes immediately before, there is no case wherethe self-fluxing alloy coating is cracked.

The others, in the above described respective embodiments, as thespecific example, production in the factory of the boiler furnace panel10 and obtaining in construction site of the boiler tube 80 aredescribed, however, there may be the reverse. That is, like the abovedescription, also production of the boiler tube 80 in the factory, andobtaining the boiler furnace panel 10 in the construction site areappropriately performed, though the detailed description as beingrepetition is omitted.

1. An alloy coated boiler part for welding, comprising, before welding:a base material body, and a coating which coats the base material body,the coating composed of an alloy material comprising Ni and Cr in totalover a half proportion of the alloy material, the coating including aweld-area coating composed of said alloy material in which B is 0.1weight % or less and Si is 0.5 weight % or less said weld-area coatingbeing positioned at an end portion subjected to welding and the vicinitythereof, and a non-weld-area coating composed of said alloy material inwhich the contents of B and Si are in the range of 1 to 5 weight %respectively.
 2. The alloy coated boiler part according to claim 1,wherein said weld-area coating covers a region from the end portionsubjected to the welding and positions apart from the end portions by 15to 50 mm.
 3. The alloy coated boiler part according to claim 1 or claim2, wherein said alloy materials for the weld-area coating is composed ofsuper alloy materials corresponding to JIS G 4901, 4902-NCF 625, andsaid alloy materials for the non-weld-area coating is composed of nickelself-fluxing alloy materials corresponding to JIS H 8303-SFNi 4, andthickness ratio between said the weld-area coating and said thenon-weld-area coating is set to 1.2 to 2.0:1.
 4. The alloy coated boilerpart according to claim 1 or claim 2, wherein said alloy coated boilerpart is a boiler furnace panel or a boiler tube.
 5. The alloy coatedboiler part according to claim 1 or claim 2, wherein said alloy coatedboiler part is a boiler furnace panel in which a tube material and aplate material are joined alternately, and a notch is formed, at the endportion of said plate material.